Tile formation and development of electrostatic latent images on surfaces of photoconductive imaging members, commonly referred to in the art as photoreceptors, is well known. In these systems, and in particular in xerography, the xerographic plate (or drum or belt) containing a photoconductive insulating member is imaged by uniformly electrostatically charging its surface, followed by exposure to a pattern of activating electromagnetic radiation, such as light, which selectively dissipates the charge in illuminated areas of the photoconductive layer causing a latent electrostatic image to be formed. The latent electrostatic image can then be developed with developer compositions containing, for example, toner particles, optionally combined with carrier liquid or particles. This is followed by transferring the image to a suitable substrate such as paper. This process requires the photoconductive member to photogenerate and transport charge, thereby neutralizing the charge on the surface.
Examples of photoconductive members include members comprised of inorganic materials and organic materials, layered devices of inorganic or organic materials, composite layered devices containing photoconductive substances dispersed in other materials, and the like. Current layered organic photoreceptors have a substrate layer and two active layers: (1) a thin charge generating layer containing a light-absorbing pigment, and (2) a thicker charge transport layer containing electron donor molecules in a polymer binder. The electron donor molecules (e.g., triaryl diamines) provide hole or charge transport properties, while the electrically inactive polymer binder provides mechanical properties. The charge transport layer can alternatively be made from a charge transporting polymer such as poly(N-vinylcarbazole), polysilylene or polyether carbonate, wherein the charge transport properties are incorporated in the mechanically strong polymer. These photoconductive members can optimally include a charge blocking and/or adhesive layer between the charge generating and the conductive layers. Additionally, they may contain protective overcoatings and the substrate may comprise a nonconductive and a conductive layer. Additional layers to provide special functions such as incoherent reflection of laser light, dot patterns for pictorial imaging or subbing layers to provide chemical sealing and/or a smooth coating surface may also be employed.
In a preferred photoreceptor, the photoreceptor surface is charged to a negative polarity by a corona device and discharged by visible or infrared light or radiation to form a charge pattern or image. The light is primarily absorbed by the pigment in the charge generating layer which photogenerates the charge carriers. The positive charges in this pigment or charge generating layer are injected into the charge transport layer and transported to the surface of the charge transport layer, thereby discharging the layers.
Generally, pigments used in the charge generating layer can be classified into two classes on the basis of their photogeneration mechanisms: (1) intrinsic and (2) extrinsic. In intrinsic pigments, the positive and negative charges are separated directly and transported internally, without the assistance of a charge transporting species, to the surface of the charge generating layer. Selenium, selenium tellurium alloys, and arsenic selenium are examples of intrinsic inorganic pigments. Examples of organic intrinsic pigments are phthalocyanines.
With extrinsic pigments, charges are not readily separated but require charge transporting material or molecules in the vicinity of the photogeneration process for charge separation. Hence by themselves extrinsic pigments are very insensitive to photogeneration. Examples of extrinsic organic pigments are perylene diamine pigments. Extrinsic inorganic pigments include cadmium sulphate and zinc oxide.
U.S. Pat. No. 3,904,407 to Regensburger et. al. discloses multilayer electrophotographic elements including a perylene pigment charge generating layer, a transport layer and a conductive substrate. These perylene pigments can be vacuum-deposited to form high sensitivity charge generation layer. U.S. Pat. Nos. 3,871,882, 4,419,427, 4,578,333, 4,578,334, 4,587,189 and 5,019,473 disclose multilayer imaging members incorporating a perylene-3,4,8,10-tetracarboxylic acid imide derivative pigment charge generation layers wherein the pigment is dispersed in a polymeric binder or vacuum deposited. In all these disclosures, claiming high sensitivity perylene pigment charge generation layers, the charge transport layer consists of solutions or dispersions of arylamine electron donor molecules in a polymer binder.
The sensitivity of a layered device depends on several factors: (1) the fraction of the light absorbed, (2) the efficiency of charge photogeneration within the pigment crystals, (3) the efficiency of injection of photogenerated charge carriers into the transport layer and (4) the distance the injected charge carrier travels in the transport layer in the time between the exposure and development steps. The fraction of the light absorbed can be maximized by increasing the thickness of the generator layer and/or the concentration of pigment in the generator layer. The distance the charge carrier travels in the transport layer can be optimized by the selection of the charge transporting material and by the concentration of the charge transporting active molecular sites. However, the efficiency of photogeneration and injection can be interactive in that both processes depend on both the pigment and the transport material. The photogeneration efficiency with some pigments depends upon the presence of charge transporting material on the surface of and therefore in contact with the pigment. These pigments are extrinsic as distinguished from intrinsic pigments whose photogeneration efficiency is high even in the absence of such transport material.
The layered devices fabricated from extrinsic pigments may be less sensitive in the following situations: (1) A two layer device in which the charge generator consists of pigment loading in high enough concentration to assure particle contact in an inactive binder and the transport layer is fabricated from a dispersion of charge transporting molecules in an inactive binder. The charge transporting molecules of the transport layer may not be soluble in the binder used for the generator layer. If the generator layer pigment is extrinsic, only that part of the generator layer in contact with the transport molecule is sensitive to light. This would be the pigment located in a very narrow region in the very top part of the generator layer. The exposure or erase light absorbed in the pigment located below this region of the generator layer is essentially wasted. (2) A two layer device whose generator layer is fabricated by sublimation of the extrinsic pigment and whose transport layer is fabricated from a dispersion of charge transporting molecules in an inactive binder which does not penetrate the generator layer. A thin pigment layer located in the very top part of the generator layer is in contact with the charge transporting molecules and the light absorbed in this portion of the generator layer produces free carriers with high efficiency. The exposure or erase light absorbed in the pigment located below this region of the generator layer is essentially wasted. (3) A two layer device containing a generator layer either fabricated from extrinsic pigments in a binder or fabricated from sublimed extrinsic pigments and a transport layer containing a charge transporting polymer that cannot readily diffuse into the generator layer.
There is no certainty that a pigment that seems sensitive in a device employing a charge transport layer containing a solid state solution of charge transport molecules in a polymer binder will have good sensitivity when employed in conjunction with a charge transporting polymer. One of the architectural advantages of multilayered organic photoreceptors is that when fabricated on semitransparent substrates, the erase light can be incident from the substrate side. This option is not easily available for conventional multilayered devices employing extrinsic pigments, as the erase lamp intensity has to be extremely high.
As discussed, the photogeneration efficiency of extrinsic pigments such as benzimidazole perylene by itself is very low (0.01 charge carriers per absorbed photon). A proposed explanation for this is that the absorbed photons produce bound charge pairs (excitons) that recombine or relax to the ground state with very inefficient production of free charge carriers. The presence of electron donor molecules such as those in the charge transport layer enables the excitons at the pigment molecule interface to dissociate by electron transfer from the electron donor molecules, increasing photogeneration efficiency. Thus, the photogeneration efficiency of benzimidazole perylene pigment in the presence of a triphenyl diamine such as N,N'-diphenyl-N,N'-bis (3 methylphenyl)-1,1'-bisphenyl-4,4'diamine, is very high (e.g., 0.3 to 0.6 charge carriers per absorbed photon).
The photogeneration efficiency of a benzimidazole perylene charge generation layer used in conjunction with a charge transport layer composed of a charge transporting polymer such as poly (N-vinylcarbazole), polysilylenes, polyarylamines and others including those described in U.S. Pat. Nos. 4,618,551, 4,806,443, 4,806,444, 4,818,650, 4,935,487, and 4,956,440, is very low when compared to a triphenyldiamine solution charge transport layer used in conjunction with a benzimidazole perylene. This is thought to be because the electron donor moieties in the poly(N-vinylcarbazole) polymer cannot and therefore do not penetrate into the charge generating layer as do the small molecules of triphenyldiamine.
This therefore places a new requirement on the properties of charge transport layer materials. With the use of extrinsic pigments like perylene diamines, a transport polymer material such as poly(N-vinylcarbazole) cannot be employed if it does not meet the aforementioned photogeneration requirements. This produces a particular problem in situations where polymeric charge transport layer materials such as poly(N-vinylcarbazole) are preferred over two phase charge transport layers formed by molecular solutions or dispersions of electron donor molecules in a binder. An example of such a situation are in photoreceptors subject to inks with liquid carriers such as Isopar.RTM. which attack two phase charge transport layers.